Question about fast charging.

Hello guys! I just had a question about the fast charging in the nexus 6. I recently flashed the Android M preview to my nexus, used it for a couple days, and then reflashed 5.1.1 back to complete stock. It got me to wondering about if the fast charge feature was software related or all hardware. Like, if I installed an AOSP rom like cyanogenmod or another one, does it affect the phones ability to fast charge? How does that work? Is that built into the chipset or is it part of the phones kernel?

Sent from my Nexus 6